The present invention relates to a method of forming a molded article and, more particularly, to a method of forming and a molded article having a passage therein which has a cross-sectional dimension adjacent an end which is less than at a location which is spaced from that end.
Double tapered articles are known in which the cross-sectional dimension of the article, e.g. the diameter, decreases toward each of the ends. These are typically articles of metal or other materials which are formed by grinding or sanding the ends of a rod to define the decreasing cross-sectional dimension and/or by compressing or swaging a tube toward one or both of its ends to form at least one and sometimes opposite tapered ends. Such compressing or swaging of tubing is, for example, disclosed in my U.S. Pat. No. 3,274,816.
In certain circumstances it would be advantageous to form an elongate one piece shaft by the molding of a composite polymeric material about a mold and in which the mold, and hence the passage in the shaft which remains after the removal of the mold, has a double taper, e.g. is tapered toward each end. However, this previously has not been possible because it was not possible to remove the mold about which the shaft was formed once the polymeric material used to form the shaft has been set or cured. This is because the cross-sectional dimensions at the tapered ends of the passage which is formed by the mold are smaller than in between the ends and, therefore, block the movement of the mold, which has a larger cross-sectional dimension between its ends, out of the passage. Such shafts of varying internal cross-sectional dimensions would be highly desirable, for example, in a javelin where the cross-sectional dimensions of both the interior and exterior of the shaft desirably vary over the length of the javelin for proper weight, balance and flexing qualities.
This problem has been solved in the present invention. The present invention makes it possible to form a one piece, molded, polymeric shaft having a passage therethrough which is formed by the mold, and which varies in cross-sectional dimension over its length so that these dimensions are less at the ends of the shaft than at a location spaced from the ends. Yet the mold about which the shaft has been formed is capable of easy removal from such shaft passage after the article has been formed and set.
In one principal aspect of the present invention, a method of forming a molded article having a passage therein which has a cross-sectional dimension adjacent an end which is less than at a location which is spaced from such end, comprises providing an elongate substantially rigid mandrel having a substantially rigid surface thereon and which surface has a given shape, and covering a substantial portion of the rigid surface of the rigid mandrel with a flexible mandrel. The flexible mandrel has an inner surface which assumes the shape of the rigid surface of the rigid mandrel when the flexible mandrel is positioned thereon, and an exterior surface on the flexible mandrel which has a cross-sectional dimension which is less adjacent an end thereof than at a location which is spaced from the end. The flexible mandrel is covered with a material which is to form the molded article, with the rigid and flexible mandrels defining a mold about which the molded article is formed by the material. The material is set to form the molded article about the mandrels with the mandrels defining the passage in the molded article, and the mandrels are removed from the molded article from the end of the passage which has the cross-sectional dimension which is less than at the location which is spaced from such end.
In another principal aspect of the present invention, the rigid mandrel is removed from the passage first and then the flexible mandrel is removed from the passage.
In still another principal aspect of the present invention, the rigid mandrel is metal.
In still another principal aspect of the present invention, the rigid mandrel is an elongate solid rod.
In still another principal aspect of the present invention, the rigid mandrel is elongate and of uniform cross-sectional dimension over a major portion of its length, and is preferably cylindrical.
In still another principal aspect of the present invention, the flexible mandrel stretches when it is being removed from the passage in the molded article.
In still another principal aspect of the present invention, the flexible mandrel is formed of silicone rubber.
In still another principal aspect of the present invention, the flexible mandrel is a sleeve which is pulled over the rigid mandrel to cover its rigid surface.
In still another principal aspect of the present invention, the flexible mandrel has a high thermal coefficient of expansion so that it exerts a pressure on the wall of the passage and the rigid surface of the rigid mandrel during the setting of the material.
In still another principal aspect of the present invention, the material is heated during setting.
In still another principal aspect of the present invention, the material is constrained on the side thereof opposite the mandrels during setting.
In still another principal aspect of the present invention, the material is constrained by wrapping with a tight stretch film.
In still another principal aspect of the present invention, the material includes an epoxy resin.
In still another principal aspect of the invention, the method is for molding the shaft of a javelin.
In still another principal aspect of the present invention, an article of manufacture comprises a molded elongate single piece shaft formed of a polymeric material with the shaft having first and second ends. A passage opens to the first end and extends from the first end toward the second end and over a substantial portion of the length of the elongate shaft. The passage is formed from and defined by the mold upon which the shaft is molded and after the mold is removed, and the passage has a cross-sectional dimension which is less at the first and second ends than between the ends.
In still another principal aspect of the present invention, the shaft also has an external surface having a cross-sectional dimension which is less at the first and second ends than between the ends.
In still another principal aspect of the present invention, the polymeric material includes an epoxy resin.
In still another principal aspect of the present invention, the polymeric material comprises a fabric selected from the group consisting essentially of fiberglass, carbon fibers, ceramic fibers and mixtures thereof which is impregnated with an epoxy resin.
In still another principal aspect of the present invention, the shaft is the shaft of a javelin.
These and other objects, features and advantages of the present invention will be more clearly understood through a consideration of the following detailed description.